More than 86% of electricity in the U.S. is produced in thermoelectric power generating plants, most of which use coal, natural gas, or nuclear power to generate thermal energy. The thermal energy drives steam turbines to produce electrical power, and typically more than 60% of the original energy is wasted and carried away as low-grade heat. Operators must remove this heat, and 99% of baseload thermoelectric plants in the U.S. use water-cooled systems, or wet cooling, to do so. As a result, wet-cooling systems at power plants currently account for 41% of all fresh water withdrawals in the U.S.
Since availability of fresh water resources is increasingly strained by drought and growing demand, and potential climate change impacts add uncertainty to the quality and quantity of future water supplies, dry-cooling systems (air cooled condensing) have therefore received increased attention. In these systems, however, the air temperature tends to be warmer than the fresh water temperature, and air has a substantially smaller cooling capacity compared with that of water. As a result, current dry-cooling technologies are less effective in cooling the steam in power plants, thus resulting in a reduction in the efficiency of power generation compared with that for water cooled generators. Specifically, during hot ambient temperatures, the efficiency of an air cooled condenser is even lower because of the temperature difference between the air and the steam is low. As a result, power plants are overdesigned with respect to air cooled condenser cooling capacity, so they can handle the highest ambient temperatures. This dramatically increases the capital cost of dry cooling systems.
To enhance the thermal efficiency of dry cooling systems, research and development has focused on heat transfer enhancement techniques for air cooled condensers. There are a wide variety of strategies, and generally can be classified in two broad categories: improvement of air-side heat transfer and enhancement of steam-side heat transfer.
The general concept of the improvement of air-side heat transfer is to modify the existing structure of air-side fins or create a new mechanical structure to have a better interaction with the feeding air, thus a higher heat transfer coefficient for the air-side convection. Conventional enhancement techniques, such as multi-louvered, ribbed and the slit-fin transport enhancement technique, operate by frequently disrupting and restarting the boundary layers, yielding a high heat transfer coefficient. While these approaches can reduce air-side thermal resistance, the resulting pressure drop increase negates the net plant performance gain. Recently, enhancement of heat transfer through increasing the air fluid mixing has also received increased attention, such as fluid-structure interaction using oscillating reeds, a piezo-actuated structure inserted into air-flow channels. The motion of the reeds generates vortical flow structures and increases fluid mixing. Such mechanical mixing techniques are still in their infancy for dry cooling applications.
In principle, there are a few ways to enhance steam-side or condensation heat transfer. One way is to increase the area of the condensing surface, and another is to reduce the thermal resistance due to the condensate film formed on the surface. In practice, a combination of these two methods is employed simultaneously to develop high-performance condenser surfaces. Copper metal with a hydrophobic coating has experimentally been shown to have a very high heat transfer coefficient (nearly five times higher than for an untreated surface), however, further studies need to demonstrate the thermal performance for steel surfaces including those used in power plants, as well as its reliability in long-term service life.
Although the methods of enhancing thermal efficiency of dry cooling systems provide some increase in efficiency, it would be beneficial to provide dry cooling with a cool storage system to provide enhanced efficiency.